The present invention relates to a method of making up a digital multiplex to be transmitted or broadcast, said multiplex being made up of packets originating from a plurality of sources with at least some of them being derived from respective packet precursors from the sources by application of a treatment selected from treatments belonging to a catalog of stored treatments. The term "packet precursor" is used to designate data already distributed in packets, but in a form that is not yet that in which the packets are subsequently transmitted or broadcast.
The invention is suitable for use when digital packets, originating from different sources, should each be subjected in real time to a predetermined treatment among a plurality of treatments before transmission or broadcast as a continuous stream. However an important, but non-exclusive, application of the invention lies in making up a digital multiplex of the kind that may be called X/packet, as defined by that portion of the document "Specification of the D2-MAC packet system" of September 1985 that is concerned with the digital portion of a television signal, the multiplex possibly being used alone, for instance for digital TV, or combined with an analog portion for instance in a D2-MAC/packet signal.
In the D2-MAC digital multiplex signal, the stream of bits for transmitting and broadcasting sound and data is split up into packets of 751 bits each, each packet having a useful working portion and also a 23-bit address enabling the origin of the packet to be identified. Prior to multiplexing with the analog signal and depending on its origin and/or nature, each packet is subjected to different treatment.
In the particular case of D2-MAC/packet, certain treatments are performed on all of the packets: packet headers, containing the addresses, are protected by inserting a Golay code; the packets are interlaced.
Other treatments are applied to some packets only, for example those belonging to the zero channel. Still in the case of D2-MAC/packet, packets representing data are protected by a cyclic redundancy check (CRC) and by a Golay code. Finally, packets corresponding to a program having conditional access are scrambled.
Treatments also need to be performed on packets in the case of a signal that may be called a full-time X/packet signal, e.g. the signal obtained by applying the method described and claimed in EP-A-0,552,099.
For greater simplicity, the description below relates essentially to generating a digital multiplex signal for incorporation in a D2-MAC/packet signal; however this is an example only.
An advantageous solution consists in performing all the treatments in the same processing unit, which receives messages from packet sources. These messages are referred to below as "packet precursors" to show clearly that they are not packets in their final form for broadcasting. This removes the need to provide each source of packets that are to be subjected to the same treatment with the means for performing that treatment.
The approach that comes to mind consists in sending an instruction to the processing unit on each arrival of a packet precursor that is intended to give rise to a packet for broadcasting, with the instruction identifying the packet and the treatment that is to be applied to the packet. This means that time coherence must be maintained for changes that take place in the sources and for changes that take place in the processing unit.